Cryolite resistant refractory

ABSTRACT

A barrier comprising: 
     a) between about 75 w/o and about 95 w/o alumina-silica refractory grain, and 
     b) between about 2 w/o and about 10 w/o of a sealant selected from the group consisting of zinc borate; sulfur; R1 Al x/2  Si y/2  O z/2  and R2 Si b/2  S c/2  C d/2  O e/2  H f/2  wherein: 
     i) R1 is selected from the group consisting of Na, K, and Li, and mixtures thereof, 
     ii) x,y and z are integers between 0 and 16, 
     iii) R2 is selected from the group consisting of Ca, Sr and Mg, and mixtures thereof, and 
     iv) b,c,d,e and f are integers between about 0 and 16.

This application is a continuation of application Ser. No. 08/311,810 filed on Sep. 26, 1994, now abandoned.

BACKGROUND OF THE INVENTION

Conventional virgin aluminum production typically involves the reduction of alumina which has been dissolved in a cryolite-containing electrolyte. The reduction is carried out in a Hall-Heroult cell containing a carbon anode and a carbon cathode which also serves as a container for the electrolyte. When current is run through the electrolyte, liquid aluminum is deposited at the cathode while gaseous oxygen is produced at the anode. Because the heat flux of the cell must be strictly maintained, insulating materials, such as dense firebrick, are used under the carbon cathode container.

Although cryolite is quite useful in dissolving alumina, it nonetheless compromises the long term life of the cell. Upon electrolysis, the cryolite ionizes into highly reactive species, in particular, sodium and fluorine ions and aluminum radicals. Over time, these species pass through the carbon cathode containment and contact the firebrick. Upon contact, the species attack the firebrick and gradually degrade its physical integrity to the point where the firebrick must be replaced.

Previous attempts to solve this problem have met with mixed success. Vapor barriers such as steel sheets have been placed between the cathode and the firebrick. However, these sheets are eventually consumed by the cryolite and produce contamination. Glass sheets have been similarly placed in an attempt to produce a viscous liquid barrier between the cathode and the firebrick, with similar results. Other efforts have used castables and dry vibrated cements ("DVC") such as mullite as the protective barrier. Although these materials react with the cryolite to produce a relatively stable assemblage, the silica and alumina contained therein are highly susceptible to attack from the ionized cryolite species. One effort to improve upon high alumina castable barriers, U.S. Pat. No. 4,126,474, added barium to the castable. Although this effort improved cryolite resistance, barium is a heavy metal and so carries with it environmental concerns.

Accordingly, it is the object of the present invention to provide a dry vibrated cement ("DVC") or castable refractory which offers superior resistance to cryolite penetration without posing environmental concerns.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a cryolite-resistant barrier comprising:

a) between about 75 weight percent ("wt %") and about 95 w/o alumina-silica refractory grain, and

b) between about 2 w/o and about 10 w/o of a sealant selected from the group consisting of zinc borate; sulfur; R1 Al_(x/2) Si_(y/2) O_(z/2) and R2 Si_(b/2) S_(c/2) C_(d/2) O_(e/2) H_(f/2) wherein:

i) R1 is selected from the group consisting of Na, K, and Li, and mixtures thereof,

ii) each of x, y and z are integers between 0 and 16,

iii) R2 is selected from the group consisting of Ca, Sr and Mg, and mixtures thereof, and

iv) each of b,c,d,e and f are integers between 0 and 16.

In preferred embodiments, the alumina-silica refractory is mullite and the sealant is an alkali feldspar, (Na,K)AlSi₃ O₈.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

It has been unexpectedly found that small amounts of sodium, potassium or calcium-based sealants (e.g., a feldspathic-type material) imparts the necessary bonding, sealing, penetration resistance and erosion resistance to alumina-silica cement mixtures (e.g., a mullite-based cement) to provide a cryolite-resistant refractory product more capable of withstanding the rigorous conditions of the aluminum reduction cell.

Without wishing to be tied to a theory, it is believed that the sealant increases resistance to cryolite penetration in two ways. First, at typical operating temperatures, the sealant forms a viscous glass which seals the pores of the alumina-silica refractory from cryolite penetration. It is believed the large ionic radii of the cations in the alkali- and alkaline earth-based sealants increase the viscosity of the liquid at the cryolite/refractory interface, thereby making a thin semi-permeable film between the refractory's pores and the corrosive components of the reduction cell (e.g., cryolite). Second, the glass also bonds together the refractory grains to reduce erosion.

However, the desirability of increasing the sealant concentration in the barrier is limited by the sealant's less attractive characteristics, namely reactivity and refractoriness. Hence, it is contemplated that selection of the proper sealant concentration depends upon a delicate balance between the refractoriness, reactivity, and viscosity of the sealant.

Adding sealants to alumina-silica refractories poses the danger that the refractory and the sealant will react with one another to form new, less cryolite-resistant compounds. Accordingly, the sealant should have a low chemical reactivity towards the refractory. In this respect, feldspar is highly advantageous because it does not react with mullites to form new compounds. Moreover, the other compounds present in the cell in significant amounts (e.g., sodium, aluminum and silica) are also the constituents of feldspar. Therefore, feldspar will not react with any of the predominant compounds in the furnace.

Because the sealants contemplated for use in the present invention are typically less refractory than alumina-silica refractories, their inclusion in the alumina-silica based cements lowers the overall refractoriness of those cements. Thus, the concentration of sealant should be selected to provide a seal for the refractory barrier against the cryolite without significantly lowering the refractoriness of the barrier.

Experiments have shown that adding between about 2 w/o and about 10 w/o of certain sealants (such as feldspar) to the alumina-silica cements provides the desired bonding, sealing, erosion and penetration resistance. It is therefore contemplated that other useful cements of the present invention are generally described by R1 Al_(x/2) Si_(y/2) O_(z/2) and R2 Si_(b/2) S_(c/2) C_(d/2) O_(e/2) H_(f/2) wherein:

i) R1 is selected from the group consisting of Na, K, and Li, and mixtures thereof,

ii) x,y and z are integers between 0 and 16,

iii) R2 is selected from the group consisting of Ca, Sr and Mg, and mixtures thereof, and

iv) b,c,d,e and f are integers between 0 and 16.

More preferably, R1 is Na, K or mixtures thereof, x/2 is between about 0.5 and about 2, y/2 is between about 0.5 and about 8, and z/2 is the sum of 0.5+0.75 w+y; and R2 is Mg, Ca or mixtures thereof, b/2 is between about 0 and about 4, c/2 is between about 0 and about 1.5, d/2 is between about 0 and about 1, and e/2 and f/2 are those values which chemically balance the valence of the sealant.

In some embodiments wherein the sealant is described by R1 Al_(x/2) Si_(y/2) O_(z/2), the ratio of y to x is more than 2:1, preferably more than 3:1, more preferably more than 4:1.

In preferred embodiments of the present invention, the sealant is a sodium-containing compound, more preferably feldspar. When feldspar is used, it generally comprises between about 1 w/o and about 10 w/o of the cement, preferably between about 1.5 w/o and 6 w/o. It has been found that resistance to cryolite penetration typically increases with feldspar concentration up to about 6 w/o feldspar. From about 6 w/o to about 10 w/o feldspar, resistance is apparently maintained or slightly degraded. Preferably, the feldspar grains are -200 US mesh, i.e., less than about 75 microns.

In some embodiments, nepheline ((Na,K)AlSiO₄), is used as the sealant. However, although nepheline addition does create a viscous glass (thereby increasing cryolite resistance), it does not do so as well as feldspar.

It has also been found that certain calcium-based compounds impart significant resistance to cryolite penetration to alumina-silica cements. These calcium compounds include CaCO₃, CaSO₄ :2H₂ O, and Ca(OH)₂. Other compounds which have been found to impart cryolite resistance to alumina-silica refractories include zinc borate and sulfur.

It is believed that calcium salts are effective sealants because they react with the alumina-silica refractory at typical furnace temperatures to form feldspathic CaO--Al₂ O₃ --SiO₂ compounds and glasses which seal the refractory from penetration of constituents of the aluminum bath.

It is also contemplated that certain magnesium-based compounds (such as MgO and talc), potassium-based compounds (such as potassium-rich feldspar), may impart cryolite resistance to alumina-silica cements because of the likelihood of their forming viscous glasses. In the same manner as calcium based compounds, MgO, Mg(OH)₂ and talc (magnesium silicate) react to form a 3-component sealant of MgO--Al₂ O₃ --SiO₂.

It is believed that zinc borate forms a low melting point glass which acts as a seal.

In general, the sealants of both the DVC and cast embodiments of the present invention are present in a grain size finer than 150 mesh (less than 106 microns), preferably finer than 325 mesh (less than 45 microns). In preferred embodiments utilizing feldspar, the feldspar is less than 200 mesh (less than 75 microns).

Some of the castables containing candidate sealants exhibited acceptable chemical resistance but either unacceptable behavior in water or unacceptably large shrinkage during firing. Accordingly, it was concluded these sealants could only be used in small amounts. For example, zinc borate was examined as a sealant but exhibited a high solubility which adversely affected the flow and set of the castable. Further, castables containing at least about 6 w/o of either gypsum(CaSO₄ :2H₂ O), calcium carbonate or calcium hydroxide ((Ca(OH)₂) experienced volume changes upon firing significant enough to produce excessive cracking. Finally, mixes containing one of Ca(OH)₂, CaSO₄ :2H₂ O and zinc borate as sealants required unacceptably high amounts of water (i.e., more than about 9 w/o) to achieve sufficient flowability, and so were prone to unacceptably high porosity and shrinkage upon firing.

The alumina-silica refractory component of the present invention should provide acceptable insulation for the firebrick while maintaining cryolite resistance. It may include compounds such as mullite, pyrophyllite, calcined fireclay or kaolin, calcined bauxite, calcined alumina, and tabular or fused alumina. Preferably, alumina comprises between about 30 w/o and about 90 w/o, more preferably between about 60 w/o and about 85 w/o of the cement, while silica comprises between about 10 w/o and about 60 w/o, more preferably between about 15 w/o and about 40 w/o of the cement. In preferred embodiments, this fraction is a calcined mullite (3Al₂ O₃ :2SiO₂) grain and comprises between about 75 w/o and 95 w/o of the cement. In more preferred embodiments, Mulcoa 47, Mulcoa 60 or Mulcoa 70, each available from Combustion Engineering of Windsor Locks, Conn. is used as the refractory grain. Because the alumina-silica refractory component predominates the present invention, its grain size distribution should provide a tight, dense packing. Preferably, all of the alumina-silica grains pass through a 4 mesh screen, with at least 12 w/o less than 325 mesh (44 microns).

It has also been found that when the alumina content of the alumina-silica fines is increased by about 5 w/o alumina, cryolite resistance of the resultant barrier is increased. It is believed the increased resistance of this embodiment is due to the formation of feldspathoids. Feldspathoids are believed to be more resistant to cryolite penetration than feldspar.

In the castable embodiments of the present invention, water typically comprises between about 5.5 w/o and about 7.5 w/o of the castable, based upon the weight of the dry castable. The lower regions of this range are used when the selected alumina-silica grains have low porosity, while the upper regions are used when the selected alumina-silica grains have high porosity. However, other parameters such as temperature, humidity, dry mix moisture, mixing intensity, grain surface area variability and grain shape also affect the optimum percentage. Because higher amounts of water increase the porosity of the cast body (which in turn increases the expected shrinkage and pore size of the final refractory), wetting agents and fumed silica can be added to enhance castability and so reduce water content. Typical wetting agents include sodium or potassium pyrophosphates, orthophosphates or metaphosphates; sodium or potassium silicates; and lignosulfonates.

The cement of the present invention typically has a binder fraction which serves to bond the refractory grains together. This binder fraction may include calcium aluminate, other suitable calcium/aluminum compounds, lignin liquors, hydraulic binders, fondue cement, and phosphate-containing binders such as monoaluminum phosphates. The binder is usually present in amounts of between about 2 and 10 w/o of the cement, preferably between about 4 w/o and about 8 w/o. Typically, the cementitious binders are sized so that at least about 80% is less than 44 microns. In preferred embodiments, calcium aluminate is used as the binder in an amount of between about 2 and about 10 w/o of the cement.

The starting materials for both the DVC and castable embodiments of the present invention are usually initially processed in the same manner. Typically, the desired concentrations of the selected components are loaded into a mixer which mixes the components until the charge is completely homogenized.

In some embodiments of the present invention in which a DVC is installed in either the bottom of a Hall cell or atop insulating brick, the cement components set out below in TABLE I are mixed until homogeneous.

                  TABLE I     ______________________________________                                  PREFERRED     CONSTITUENT  GENERAL RANGE   RANGE     ______________________________________     Mulcoa 47 4 × 8                  28-30 w/o       29      w/o     Mulcoa 47 8 × 20                  23-27           25     Mulcoa 47 20F                  13-17           15     Mulcoa 47 48M                  10-12           11     Mulcoa 47 200M                  01-10           5     Fine reactive alumina                  0-6             5     Flint -200M  0-3             0     Kyanite -200M                  0-6             4     Sealing Additive                  1.5-10          --     feldspar     1.5-10          6     nepheline    3-10            0     gypsum       1.5-6           0     Ca(OH).sub.2 1.5-6           0     CaCO.sub.3   1.5-6           0     Frit         3-6             0     CaAluminate  3-6             0     ______________________________________

The selected mixture is placed on the bottom of the cell, the depth of the loose material being limited to between about 2 and about 6 inches. The mixture is then forked to promote both deairing and densification (packing). Next, a conventional surface vibrator (Bosch type) or vibrating sled contacts the mix, promoting further densification. After the surface of this densified DVC is scratched, another load of the homogenized mixture is poured onto the surface, deaired and vibrated. This procedure is repeated until the desired depth of the refractory is attained.

In another DVC embodiment, a single large load of the homogenized mixture is placed in the cell and the entire cell is vibrated until the charge packs down to the desired depth.

In another DVC embodiment, the three coarsest fractions of the Mulcoa 47 grains are replaced with Mulcoa 70 grains.

In an embodiment of the present invention in which a castable is installed, the cement components set out in TABLE II below are mixed until homogeneous (The notations "4×8","8×20", "20 F" and "200M" refer to standard mesh sizes)

                  TABLE II     ______________________________________                                  PREFERRED     CONSTITUENT  GENERAL RANGE   RANGE     ______________________________________     Mulcoa 47 4 × 8                  47-50 w/o       48      w/o     Mulcoa 47 8 × 20                  05-10           7     Mulcoa 47 20F                  05-10           7     Mulcoa 47 200M                  15-28           17     Fine reactive alumina                  00-06           5.15     Kyanite -200M                  00-04           0     CaAl cement  02-10           5     Silica fume  03-05           4.8     Wetting agent                  0.01-0.5        0.05     Sealing Additive                  1.5-10          --     feldspar     1.5-10          6     nepheline    3-10     gypsum       1.5-6     Ca(OH).sub.2 1.5-6     CaCO.sub.3   1.5-6     Frit         3-6     zinc borate  1.5-3     ______________________________________

The homogenized mixture is placed into a suitable mixer, preferably a paddle type mixer whose blades rotate through the mixture (as opposed to the tumbling action found in conventional mortar-type mixers). Next, a preselected amount of water is added to the mixer during mixing. When the water and mixture become homogeneous, the castable is removed from the mixer and placed into the cell. At this time, the castable is vibrated with a vibrator, preferably either a form, immersion or pencil vibrator. If the amount of castable placed in the cell is found to be insufficient, additional castable should be added as quickly as possible to insure against stratification in the final product.

In another castable embodiment, the three coarsest fractions of the Mulcoa 47 grains are replaced with Mulcoa 70 grains.

The present invention may be used in any application generally suitable for refractories, including DVC's, castables, ramming mixes, bricks and mortars. Preferably, it is used as a liner in an aluminum production reaction chamber.

EXAMPLE I

Castable and DVC mixtures comprising the constituents set out in TABLE III below along with typical additives set out in TABLES I and II were selected for penetration and erosion testing. Castable mixes are designated as "C" while DVC mixes are designated as "D". The sealant fraction is set out in weight percent. "M47" means all of the alumina-silica grains are Mulcoa 47, while "M70" means the three coarsest fractions of the alumina-silica grains are Mulcoa 70.

                  TABLE III     ______________________________________     Mix     Alumina-Silica                          Sealant     ______________________________________     Mix CK  M70          3% Barite     mix C1  M70          3% Gypsum     Mix C2  M70          3% Whiting     Mix C3  M70          3% Ca(OH).sub.2     Mix C4  M70          3% Feldspar     Mix C5  M70          6% Feldspar     Mix C6  M47          10% Feldspar     Mix C7  M70          3% Zincborate     Mix C8  M70          3% C8722 Frit     Mix C9  M70          3% Sulfur     Mix D1  M47          10% Nepheline-Fumed Silica     Mix D2  M47          10% Nepheline-A12 Alumina     Mix D3  M47          3% Feldspar     Mix D4  M47          6% Feldspar-Fumed Silica     Mix D5  M47          6% Feldspar-A12 Alumina     Mix D6  M47          10% Feldspar     Mix D7  M70          3% Feldspar     Mix D8  M70          6% Feldspar     Mix D9  M70          10% Feldspar     Mix D10 M70          3% Calcium Aluminate Cement     ______________________________________

For the castable embodiments, 9"×4.5"×2.5" straight bricks were cast with the required amounts of water. The bricks were allowed to cure overnight covered in plastic and then removed from the mold and placed in a hot air dryer to remove the pore water. Samples were then fired at about 1500° C.

For the DVC embodiments, 9"×4.5"×2.5" straight bricks were pressed. The dry material was mixed with about 3 w/o water and a starch binder and hydraulically pressed on a uniaxial 300 ton press so the same density was achieved with a "Bosch" vibrated sample, to simulate actual installed properties. After pressing, the samples were placed in a hot air dryer overnight to dry. The samples were fired to about 1500° C.

Next, the castings were core drilled to about a 4 inch depth and a 1 inch diameter to provide a cup capable of containing a cryolite-sodium fluoride liquid.

About 0.5 grams of quarter inch diameter aluminum pellets were placed into the bottom of the cup. Next, a premixed mixture of about 70 w/o cryolite and 30 w/o sodium fluoride was poured and tamped into the cup until it reached a level approximately 1/8 inch from the top of castable. Approximately 80 grams was required to fill the cup. The castable top and open cup area were then covered with a piece of fireclay split brick having the same cross-section dimensions as the castable sample. The samples were then placed in a nitrogen-purged furnace and heated to 950° C. at the rate of 50° C. per hour. The furnace was held at 950° C. for 100 hours before it was shut off. The samples then cooled while remaining in nitrogen-purged furnace. Once cooled, the samples were removed from the furnace and cut in half transversely through the center of the cup cavity.

Using millimeter rule, the amount of erosion was determined by measuring the extent of lost casting, while the amount of penetration was determined by measuring the extent of casting discoloration. Both measurements were made outward from the cup center to the points of maximum erosion and penetration on either side of the cup cavity. The original cup cavity width (1.0 inch or 25.4 mm) is subtracted to determine the extent of destruction and penetration.

The castings were found to have the following resistances:

    ______________________________________                      Erosion(in)                              Penetration(in)     ______________________________________     Mix C4 Mix CK (M70-3% Barite)                        0.22      0.395     Mix C1 (M70-3% Gypsum)                        0.167     0.397     Mix C2 (M70-3% Whiting)                        0.21      0.40     Mix C3 (M70-3% Ca(OH)2)                        0.21      0.399     Mix C4 (M70-3% Feldspar)                        0.157     0.34     Mix C5 (M70-6% Feldspar)                        --        0.307     Mix C6 (M47-10% Feldspar)                        0.274     0.278     Mix C7 (M70-3% Zinc borate)                        0.20      0.414     Mix C8 (M70-3% C8722 Frit)                        0.15      0.37     Mix C9 (M70-3% Sulfur)                        0.144     0.406     Mix D1 (M47-10% Nepheline-                        0.293     0.444     Fumed Silica)     Mix D2 (M47-10% Nepheline-A12                        0.197     0.335     Alumina)     Mix D3 (M47-3% Feldspar)                        0.30      0.441     Mix D4 (M47-6% Feldspar-Fumed                        0.232     0.390     Silica)     Mix D5 (M47-6% Feldspar-A12                        0.281     0.326     Alumina)     Mix D6 (M47-10% Feldspar)                        0.225     0.394     Mix D7 (M70-3% Feldspar)                        0.15      0.389     Mix D8 (M70-6% Feldspar)                        0.16      0.299     Mix D9 (M70-10% Feldspar)                        --        0.363     Mix D10 (M70-3% Ca Aluminate                        0.285     0.467     Cement)     ______________________________________

DVC "D" mixes with whiting, calcium hydroxide, and sulfur were prepared but not tested as samples could not be formed without excessive water for pressing. Each either cracked after firing, or could not be core drilled due to excessive friability. Similarly, castable "C" mixes cast with 6% or more zinc borate, sulfur, whiting, calcium hydroxide and gypsum were prepared but not tested because each either required excessive amounts of water to vibrate, or cracked after firing. However, mixes with 6% and more feldspar cast without any difficulty or increase in water content.

Castable and DVC barriers containing feldspar were also examined by XRD. Within the cup where the molten electrolyte was placed, Na₅ Al₃ F₁₄, CaF₂, Na₃ AlF₆, a--Al₂ O₃ and glass were detected. In the penetration zone between the molten electrolyte and the refractory, CaF₂, Na₃ AlF₆, a--Al₂ O₃ and glass were detected. Where no penetration had taken place, a--Al₂ O₃, SiO₂, mullite, kyanite, and glass were detected. There was no significant difference between the DVC and castable with respect to the phase assemblages found.

It is also contemplated that the above listed sealants and especially feldspar, would be useful in imparting similar resistance to nitride bonded-silicon carbide-based refractories. 

I claim:
 1. A cryolite-resistant refractory for use in a Hall-Heroult aluminum production cell, the refractory consisting essentially of:a) between about 75 weight percent wt % and about 95 wt % alumina-silica refractory grain, b) between about 2 wt % and about 10 wt % feldspar.
 2. The refractory of claim 1 further comprising between 2 wt % and 10 wt % binder.
 3. The refractory of claim 1 wherein the feldspar is present in an amount of about 6 wt %. 